General Charles De Gaulle rides in a motorcade through Algiers, in 1958, after an armed uprising there called for him to take control of France. Photograph: Loomis Dean/Time & Life Pictures/Getty

It is 70 years since, in the face of ferocious German bombardment, General Hering announced that Paris was an "open city" and effectively surrendered to the Nazis. This brave decision was taken on good military grounds – there was no way that the city could have withstood any serious attack by air or artillery and it is clear that massacres and much suffering were avoided. The troops of General von Küchler's XVIIth army entered Paris on 14 June 1940 at 5.30 in the morning. Two divisions advanced in tandem on the Eiffel Tower and the Arc de Triomphe. By noon, the German high command had taken up residence in the Hôtel de Crillon on the Place de la Concorde. There had been no battle, or even the slightest sign of resistance. To the Germans, and especially to Hitler, this was direct proof of the general theory of the morally degenerate nature of the French.

It was at this point that an obscure French general called Charles de Gaulle entered history. Until now, De Gaulle's role in the war had been limited to overseeing the divisions that were trying to delay the German advances on the Maginot Line, which De Gaulle had already publicly identified as the main flaw in French military strategy. Within days of the final collapse of France, De Gaulle was flown by the British to London where, on 18 June, from the studios of the BBC, he made an emotional and powerful call for the French to resist the Nazi invaders. This rallying cry is now know as the Appel du 18 Juin and is a touchstone of what it means to be French; its fine phrases and cadences are as well known to all French-speaking people as any of the wartime speeches of Churchill.

The language is fiery, poetic and imperious: "Whatever happens, the flame of French resistance must not be extinguished," he thundered, "and it will not be extinguished." De Gaulle demanded that all those who did not accept the Nazi occupation must define themselves as the Free French and in doing so begin the war of resistance. With this one short speech, De Gaulle started not only the regeneration of the French-speaking world – an act Hitler thought was impossible – but changed the very shape of the 20th century.

As Jonathan Fenby points out in this finely nuanced and highly readable biography, this speech almost didn't happen. On 18 June, De Gaulle lunched with Alfred Duff Cooper, the urbane minister of information, who revealed nothing of his disquiet to the general but discreetly alerted the prime minister's office that De Gaulle could not quite be trusted. In a day of frantic media politicking worthy of Malcolm Tucker, it was finally decided that De Gaulle could speak, but the speech was edited and never went out live as De Gaulle later claimed.

This new detail uncovered by Fenby has much to tell us not only about the panic in Paris and London at this point in the war, but also reveals De Gaulle as a master of spin as much as the saviour of his nation. Indeed, as Fenby goes on to illustrate in admirably forensic detail, De Gaulle's domination of the French political landscape in the post-war period had much to do with his understanding of the popular mood and his ability to manipulate it. De Gaulle's admirers argue that he saved France three times – from the Nazis in 1945, from the attempted putsch in Algiers in 1958, and from the rioting students in Paris in 1968. In fact, as it turns out, only the first of these is true.

Having led the French government in exile throughout the war, De Gaulle resigned in 1946 in disgust at the constitution of the Fourth Republic, which he saw as favouring party political interests and a perpetually weakened state power. He was thrust back into power by the events of 1958, when an armed challenge to French government from the anti-independence pied-noirs in Algeria threatened to launch civil war in France. At the height of the tension, the general flew to Algiers and declared to the pieds-noirs, who were clamouring for him to take power: "Je vous ai compris" (I have understood you). This much was true, but it didn't mean that he sympathised with them. De Gaulle had no interest in preserving the fantasy of French Algeria and worked covertly to negotiate an end to the war and independence for Algeria. In doing so, he avoided much bloodshed, although this was not the saviour of France that his most fervent supporters had imagined, but rather a skilful politician, superbly adroit in the games of manipulation at the heart of statecraft.

By the time of the student riots of May 1968, De Gaulle was exhausted and out of touch. Most importantly, the ideology of Gaullism – a high-handed and paternalistic idea of the nation as a family ruled by a stern, morally correct guardian – was entirely disconnected from the revolt coursing through the veins of France's youth at this time. It's hard to believe at this distance, but one of the triggers for the riots that nearly launched a revolution was a dispute between university authorities and students over whether boys and girls could share dormitories. This was the culture of Gaullism in action at the most local level.

Even as the Left Bank burned, De Gaulle dismissed the insurrectionists as "a bunch of clowns" and left France for a state visit to Romania. This time, the revolt was crushed by police action and middle-class fear of anarchy. By 1969, De Gaulle was at a political dead end and irrelevant.

One of the most bracing aspects of this biography is that it debunks many of the Anglo-American myths about De Gaulle. Fenby is able to write about French political culture from the inside, and the De Gaulle he portrays escapes easy classification; he is not a fascist, nor a demagogue, nor indeed the wilfully perverse contrarian so often caricatured in the non-French world as Monsieur Non. Instead De Gaulle emerges as a heroic, even tragic figure whose certaine idée of France is defeated by each new reality that he confronts as the century evolves. President Sarkozy, who is fond of invoking De Gaulle as a model and father figure, certainly has much to learn from this tale.